Frequency hopping of a transmitted radio signal is used in a variety of spread-spectrum systems of wireless communications as it offers several advantages in both military and civilian applications. In a frequency hopping system, a coherent local oscillator is made to jump from one frequency to another, which limits performance degradation due to interference effects in a communications system, makes message interception more difficult, and lessens detrimental effects of channel collisions in multi-user systems. A description of this and other types of spread spectrum communications systems may be found, for example, in Spread Spectrum Systems, 2nd Ed., by Robert C. Dixon, John Wiley & Sons (1984) and Spread Spectrum Communications, Vol. II, by M. K. Simon et al., Computer Science Press (1985).
For military applications, frequency hopping is particularly important as the interference can take the form of signal jamming in addition to multi-path interference or multi-user interference typically present in civilian applications. The latter two forms of interference are commonly mitigated by including some form of channel equalization in the receiver, encoding and frequency domain multiplexing at the transmitter, or by adequately controlling the number of users in a given transmission area. In terms of signal jamming, however, conventional systems mitigate the effects of jamming by using either a combination of error correction coding, interleaving, and frequency hopping techniques including adaptive hopping sequences, or have to resort to scaling back the expected data rates in response to certain jamming waveforms. For example, to combat the effects of adaptive jamming waveforms, such as follower jammers which attempt to detect and adaptively follow frequency hopping of the communication system, the transmission scheme relies on the transmitter frequency hopping rate being greater than the tracking rate of the jammer.
Irrespective of the frequency hopping rate selected, conventional frequency-hopping spread spectrum systems may be easily jammed by a relatively simple jamming process, wherein several tones or Gaussian noise pulses are injected randomly among the frequency bins. This type of jamming, known as “partial-band” jamming, is recognized in the book by M. K. Simon et al., supra, to cause severe degradation in performance compared to other forms of interference. Partial-band jamming is especially damaging in the case when the jamming system (hereinafter “jammer”) is sophisticated enough to follow the signal with high probability. It may be difficult therefore to avoid performance degradation of conventional frequency hopping systems subjected to partial or full band jamming.
There is therefore a need to make frequency-hopped spread spectrum communications more robust in the presence of multiple tone or multiple Gaussian pulse jammers, partial and full band jammers.
In addition to the problems associated with providing anti-jamming capabilities, conventional wireless communication systems do not possess the ability to use the entire radio bandwidth in an adaptive and flexible manner, reflecting the highly-structured nature of legacy radio waveforms and of spectral allocation previously seen in military and civilian communications. This means that spectrum usage is often very fragmented and inefficient, with potentially large portions of the spectrum, though allocated, practically going unused.
The problem of efficient spectral usage is further exacerbated in modern wireless communications by the need to transmit high-bandwidth signals, for example combining audio and video information, or multiple data streams from multiple network users. One known method of wide-band wireless transmission is frequency domain multiplexing (FDM), in particular—orthogonal frequency division multiplexing (OFDM), which enables transmitting information from multiple users at multiple sub-carriers combined in a single OFDM signal. This method enables a multiple user access scheme wherein information from multiple users is transmitted in one contiguous block of frequency spectrum with a relatively high tolerance to multi-path interference. For a system employing frequency hopping, this results in a scheme wherein a wide-bandwidth contiguous-spectrum signal hops over the entire allocated radio bandwidth, with the aim of actively avoiding signal jamming waveforms. This approach however does not enable efficient and adaptive utilization of the entire non-contiguous and often highly-structured radio band available for transmission. Moreover, for certain types of signal jamming such as full or partial band jamming, a degradation in error rate performance or a higher required transmit power is still observed irrespective of the hopping rate of the transmitted signal.
U.S. Pat. Nos. 6,289,038 and 6,215,810 in the name of Park disclose a communication system combining FDM and frequency hopping, wherein, in order to increase robustness of the system against external interference, the same data is sent through several parallel hopping channels. A similar “frequency diversity” approach, in which replicas of the same data signal are sent over multiple frequency subbands, has been previously disclosed in a paper by E. Lance and G. K. Kaleh, entitled “A Diversity Scheme for a Phase-Coherent Frequency-Hopping Spread-Spectrum System,” IEEE Trans. Commun., vol. 45, No. 9, p. 1123-1129. However, the increased robustness to external interference in these systems is achieved at the expense of spectral utilization efficiency.
Accordingly, an object of the present invention is to provide a system and method of wireless communications wherein an initially broadband signal is divided into a plurality of narrower-band signals and transmitted over multiple frequency-hopping subbands each having a distinct frequency-hopping sequence for providing a performance gain through frequency diversity and an increased robustness to frequency jamming and mutli-path interference without sacrificing spectral utilization efficiency.
Another object of the present invention is to provide a system and method of wireless communications, wherein a broadband signal is divided into multiple narrower-band frequency-hopping subbands each of which has an adaptive frequency-hopping range spread over a full available non-contiguous band of radio frequencies for providing efficient and adaptive utilization thereof with increased robustness to frequency jamming.
It is another object of the present invention to provide a system and method of adaptive wireless communications, wherein a broadband signal is divided into multiple frequency-hopping subbands having individually adjustable subband bandwidths and adaptive modulation parameters for providing efficient and adaptive utilization of available radio bands with increased robustness to interference.
It is another object of the present invention to provide a system and method of adaptive wireless communications, wherein a broadband signal is transmitted in one or more parallel subbands frequency hopping within an adaptive RF spectrum range which is dynamically adapted to spectral distribution of external RF signals for actively avoiding external signal interference.